A need has arisen in recent years for manufacturing facilities which can provide an environment free of dust, pollen, bacteria, and other airborne particle contamination. The growing micro-electronic, drug and biomedical industries with their diminishing product geometries, have developed an increased demand for contamination free assembly methods to preclude particulate matter commonly suspended in air.
Common methods for removal of particulate contaminates flow air, which has passed through filtering elements, through the space that must be maintained free of contamination. Pre-filters of various types are often employed to collect larger particles upstream of the higher efficiency filters, thereby increasing the capability and longevity of the filtering system as a whole.
The air filtered of particulate contaminates is flowed through the critical space most commonly in a descending approach. This procedure tends to settle any elusive particulates at lower levels, preferably beneath the work area. However, systems have been employed which flow air in a horizontal path.
Modern techniques limit the air flow velocity to maintain laminar flow characteristics within the critical space of work area. This is accomplished to prevent turbulence which would interrupt the orderly passage of suspended particulates away from the work area, and which could possibly agitate particles which have become settled from the air reintroducing them into the work area. In many applications an air plenum is provided immediately preceeding the filtering elements to decrease the velocity of entering air flow to the filters, and of the particles that are therein suspended. This device allows the filter elements a better opportunity to capture particulates upon their entrance.
Early designs for accomplishing this flow of filtered air through a work area to solve the problem of particulate contamination included a blower filter assembly mounted directly over a work table to force filtered air through the space where operations were being performed. This simple and direct approach of a Clean Work Station was very economical. It could be easily assembled and moved. Its use could be easily expanded to meet the existing requirements of the user simply by purchasing more of them as was necessary. However, in practice they were less than effective if not properly used. Air contaminated with particulates surrounded the area protected in the work station. Exclusion of particulates was maintained solely by the pressure interface of the filtered air flowing into the work area and outwardly against the contaminated air surrounding. While a worker was standing and working with very little motion the clean work station would maintain a sufficiently decontaminated area. But with air flows of a velocity low enough to prevent turbulence, contaminating particles could easily be introduced into the work area from the surrounding environment. Movement into and around the work area could cause infiltration of contaminated air. Persons walking by could introduce contaminating particles to the work area from the dirty walkway between.
Also, effectiveness of air filtering was compromised in these Clean Work Stations since the filters through which air was flowed were continually being required to function in unfiltered air. It is known that the effectiveness of the filters can certainly be improved by recirculation, however this can be counter-productive for clean work stations because it results in a decreased volume of air being expelled from the work area to the surroundings. This in turn decreases the effectiveness of the pressure interface maintaining the segregation between the protected area and the surrounding air. It becomes more permeable to particulate matter. With less resistance to an influx of particulates the likelihood of contamination of the work area increases.
These deficiencies in Clean Work Station performance often resulted in the contamination of the work product. Consequently, improvement was attempted by the addition of blower filter assemblies to the room in which the stations were positioned. These would circulate filtered air into the walkway areas surrounding the work stations. However, the addition of these blower filter assemblies gave rise to new problems. It was very difficult to maintain a balance between the air flows of the work stations and the walkways to maintain the desired flow path for particle travel. The level of contamination in the areas surrounding the work stations were still much higher than satisfactory. The desultory positioning of the numerous blower-filter assemblies in the room created pockets of particle accumulation. Contamination infiltration problems still remained though at diminished levels.
In some instances these modifications actualy compounded contamination problems because persons using the Clean Work Stations believed the surrounding areas to be sufficiently free of contaminates to obviate the need for precautionarypractices. Lack of care led to contaminates being directly introduced to the work area.
The next step was to upgrade the entire room enclosing a group of work areas to a contamination free condition. This of course was a very expensive approach to the problem, requiring not only the purchase of specialized equipment but also requiring specialized architectural design and craftwork. The construction of such a structure took considerable time. First engineers and architects were required to design the structure. Building permits and inspections were then required before its construction could begin. Materials had to be purchased and delivered. It was necessary to negotiate contracts for the structure's construction, and for installation of electrical wiring, plumbing, sheetmetal duct work, and air conditioning. More importantly, the final construction was more or less permanent. Building plans had to be drawn taking into account future expansions and changes in work requirements if the Clean Room was to be versatile. This was particularly troublesome where it was necessary to build the Clean Room with another room in order to isolate a particular work area, especially for manufacturers who because of their changing product lines and changing demands could not accurately anticipate their changing needs. It was extremely expensive procedure to expand a permanently constructed Clean Room structure or to relocate it to a different area.
Though these structures were effective, it can be seen they were not desirable in many circumstances. A need, therefore, remained for an effective and flexible design of a Clean Room type structure which could be easily installed, expanded to meet changing requirements, and inexpensively fabricated, yet capable of maintaining a specified class of cleaniness for a work area.